In the field of automotive and other engines, a lean-burn engine has been put to practical use (see FIG. 1) in which, with a view to achieving better mileage, combustion at a lean air-fuel ratio, in which the percentage of air is greater than that according to the theoretical air-fuel ratio (to be hereafter referred to as a stoichiometry), and stoichiometric combustion are switched. In one example (“port injection”), fuel injection takes place near the intake port at the downstream end of the intake passage so as to achieve lean burn with an air-fuel ratio ranging from 20 to 25. In another example (“direct injection”), fuel is injected directly into the cylinder (combustion chamber) so as to produce a layered air-fuel mixture and carry out combustion in an extremely lean region with an air-fuel ratio ranging from 40:1 to 50:1. In these examples, pumping loss and thermal loss are reduced and better mileage is achieved by lean combustion; namely, by increasing the mass of air introduced into the cylinder (combustion chamber).
When stoichiometric combustion takes place, HC, CO, and NOx in the exhaust gas can be simultaneously subjected to oxidation-reduction and thus purified by a three-way catalyst installed in the exhaust passage. However, during combustion at a lean air-fuel ratio, the exhaust gas contains excess oxygen and it becomes difficult to perform NOx reduction. To counter this problem, an engine exhaust purifying apparatus has been proposed (see JP Patent Publication (Kokai) No. 2001-241320 A, for example) wherein a lean NOx catalyst is installed downstream of the exhaust passage. The catalyst stores NOx in the exhaust gas when the air-fuel ratio in the exhaust gas is lean (excess air) and discharges it when the air-fuel ratio is rich (excess fuel), thereby performing reduction, catalytic reduction, or the like. The air-fuel ratio of the air-fuel mixture is temporarily changed from lean to stoichiometric or rich at predetermined periods so as to allow the NOx stored in the lean NOx catalyst to be discharged or reduced, so that the NOx storage capacity can be restored.
The above lean NOx catalyst is adjusted depending on the air-fuel ratio in the exhaust gas. It has a high NOx storage capacity when the air-fuel ratio of the air-fuel mixture in the combustion chamber or the air-fuel ratio at the entrance to the lean NOx catalyst is approximately 17 or higher. However, its NOx purifying capacity decreases at air-fuel ratios of stoichiometry or approximately 17 and lower, so that most of the NOx in the exhaust gas is not stored and it is allowed to pass (see FIG. 2).
In order to solve this problem, a method has been proposed whereby, upon switching between combustion at a lean air-fuel ratio and combustion at a stoichiometric or rich air-fuel ratio, the duration of time in which the air-fuel ratio remains in a region where the purification or storage of NOx is not allowed is minimized.
A combustion region switching technique for reducing NOx emission is also proposed. According to this technique, when the air-fuel ratio is switched from lean to stoichiometric or rich, for example, the opening angle of the throttle valve for adjusting the mass of air introduced into the combustion chamber is reduced so as to decrease the air mass while the mass of fuel supplied (fuel injection mass) is temporarily increased and the air-fuel ratio is varied in a skipped manner so as to compensate for the transmission delay of air mass in response to the change in the opening angle of the throttle valve. The variation in engine torque caused by the increase in the fuel injection mass is compensated by retarding (delaying) the ignition timing so as to suppress the increase in exhaust emissions and the deterioration of operability. An example of an engine control apparatus that carries out such control is disclosed in JP Patent Publication (Kokai) No. 7-189799 A (1995).
Lean-burn engines aim to improve gas mileage by lean combustion. As mentioned above, in order to reduce NOx emission during lean operation, it is necessary to install a lean NOx catalyst in the exhaust passage. However, the improvement in gas mileage gained by such installation is cancelled out by the cost increase associated with the installment of the lean NOx catalyst. Thus, systems are being considered that are not equipped with lean NOx catalysts for cost reduction purposes.
FIG. 3 shows the mass of NOx emission in a lean-burn engine. As shown, when a lean NOx catalyst is installed, high NOx purification rates are obtained in regions of air-fuel ratio from stoichiometry to rich and from approximately 17 to lean, indicating the decrease in NOx emission. On the other hand, when no lean NOx catalyst is installed, NOx emissions increase in a region of air-fuel ratio from approximately 17 to lean. Thus, the air-fuel ratio region in which large amounts of NOx are emitted from the exhaust passage exit extends from stoichiometry to air-fuel ratio A, which is larger than when the lean NOx catalyst is installed. Thus, in order to reduce the NOx emission of a lean-burn engine not equipped with a lean NOx catalyst, combustion region switching technology is required whereby combustion at a stoichiometric or rich air-fuel ratio and combustion at a leaner air-fuel ratio than air-fuel ratio A can be switched while quickly passing the extended air-fuel ratio region (stoichiometry to air-fuel ratio A), in which NOx emissions increase.
When conventional technology is used for reducing NOx emission upon switching of combustion regions, the air-fuel ratio can be switched by manipulating (increasing or decreasing) the air mass and the fuel injection mass, whereby NOx emission can be reduced. However, since the air-fuel ratio needs to be changed greatly upon switching, the fuel injection mass also needs to be manipulated greatly in order to execute the switching of the air-fuel ratio. As a result, the torque variation due to the manipulation of the fuel injection mass upon switching of combustion regions increases. While the ignition timing is retarded in order to reduce such torque variation, as mentioned above, the amount of manipulation of the fuel injection mass is so large that the torque variation cannot be fully suppressed by the implementation of ignition timing retardation.
Compression ignition engines have also been proposed in which attempts are made to improve gas mileage by lean combustion, as in lean-burn engines. Compressed ignition engines are capable of operating at an ultra-lean air-fuel ratio region (air-fuel ratio of 80 or higher), which is not achievable with the conventional gasoline engines. Because flame temperature can be lowered and ignition and combustion with uniform air-fuel mixture is realized, significant decrease in NOx emission can be achieved in a compression ignition engine (see JP Patent Publication (Kokai) No. 2003-106184 A, for example).
The aforementioned compression ignition engine, however, requires highly precise ignition control. Thus, it is difficult to use the engine in high-load, high-speed conditions, and the engine is characterized in that compression ignition is conducted only in low-load, low-speed regions (see FIG. 4).
The inventors also found that, while NOx emission can be reduced during lean combustion in the compression ignition engine, the NOx emission are dependent on the air-fuel ratio. Specifically, although NOx emission are only dozens of ppm or less when the air-fuel ratio is in a region from approximately 20 to lean, hundreds of ppm of NOx are emitted in a region of air-fuel ratio from stoichiometry to approximately 20 or less (see FIG. 5). Thus, as in the lean-burn engine, combustion region switching technology is required that allows the switching between combustion at stoichiometric to rich air-fuel ratio and combustion at lean air-fuel ratio of 20 or higher in order to reduce NOx emission.
When conventional technology is applied to the aforementioned compression ignition engine, NOx emission can be reduced upon switching of the combustion regions by manipulating the air mass and fuel injection mass. However, as in a lean-burn engine not equipped with a lean NOx catalyst, the air-fuel ratio varies greatly upon switching of combustion, and so the torque variation due to fuel injection mass manipulations cannot be fully reduced by the retarding of the ignition timing.
Specifically, as explained with reference to the examples of lean-burn engines not equipped with lean NOx catalysts and of compression ignition engines, the aforementioned conventional technique does not take into consideration the cases where the amount of variation of the air-fuel ratio necessary upon switching of combustion regions becomes large. Thus, while the conventional technique can suppress the increase in exhaust emissions upon switching of combustion regions, it cannot fully suppress torque variation.
It is therefore an object of the invention to provide an engine controller capable of effectively suppressing the increase of exhaust emissions due to the increase in NOx emission and the deterioration of operability due to the development of torque variation upon switching between combustion at a stoichiometric or rich air-fuel ratio and combustion at a lean air-fuel ratio.